Abstract/Summary

Substitution of tin by indium in shandite-type phases, A3Sn2S2 with mixed Co/Fe occupancy of the A-sites is used to tune the Fermi level within a region of the density of states in which there are sharp, narrow bands of predominantly metal d-character. Materials of general formula Co2.5+xFe0.5-xSn2-yInyS2 (x = 0, 0.167; 0.0  x  0.7) have been prepared by solid-state reaction and the products characterised by powder X-ray diffraction. Electrical transport property data reveal that the progressive depopulation of the upper conduction band as tin is replaced by indium, increases the electrical resistivity and the weakly temperature-dependent (T) becomes more semiconducting in character. Concomitant changes in the negative Seebeck coefficient, the temperature dependence of which becomes increasingly linear, suggests the more highly substituted materials are n-type degenerate semiconductors. The power factors of the substituted phases, while increased, exhibit a weak temperature dependence. The observed reductions in thermal conductivity are principally due to reductions in the charge-carrier contribution on hole doping. A maximum figure-of-merit of (ZT)max = 0.29 is obtained for the composition Co2.667Fe0.333Sn1.6In0.4S2 at 573 K: among the highest values for an n-type sulphide at this temperature.